After 3 Years, European Comet Chaser to Wake Up

It has traveled 6.2 billion kilometers, got a boost by swinging past planets four times, had close encounters with two asteroids—and then it went asleep for 957 days. But on Monday, the final and most dramatic phase of the 10-year odyssey of Europe’s comet-chasing spacecraft Rosetta begins, when an internal alarm wakes it from its slumber at 10.00 GMT.

A few nerve-wracking hours will follow for scientists and controllers on the ground, as the spacecraft’s heaters warm up its systems, its startrackers get a fix, it turns its solar arrays towards the sun, and, finally, points its communications antenna toward Earth. Then, no earlier than 17.30 GMT, the Rosetta team on the ground will finally get a message telling whether all is well.

“Everything is prepared,” says Gerhard Schwehm, Rosetta’s former project scientist and mission manager at the European Space Agency (ESA). “[ESA’s control center] has done everything. The ground segment is there. Now we just wait for the first signal. Then we can get everything singing and dancing again after Rosetta’s long sleep.”

Then it’ll be time for Rosetta to close in on its target. The spacecraft is attempting to do something never done before: to catch up with a comet as it speeds toward the sun and to go into orbit around it, taking all sorts of measurements as the sun’s rays and solar wind start to heat it up and blow material off into a comet’s characteristic tail.

Previous comet missions, such as ESA’s Giotto and NASA’s Stardust, were brief flybys, catching a few hours in the life of a comet as it zipped past. Rosetta will ride along with comet 67 P/Churyumov-Gerasimenko for more than 18 months, right through its close approach to the sun, or perihelion. Previous missions were snapshots, Schwehm says; Rosetta is “a full blown movie.”

Scientific observations will begin in earnest in May, following a maneuver to synchronize Rosetta’s orbit with that of comet 67 P. As the spacecraft edges closer over the summer, different instruments will start to work: cameras and spectrometers at various wavelengths, mass spectrometers, chemical analyzers, and sounding instruments. Chris Carr of Imperial College London, the principal investigator for a suite of instruments called the Rosetta Plasma Consortium, says he hopes theirs may be the first on the spacecraft to actually detect the comet as solar wind begins to blow gas off its surface and the sun’s ultraviolet light ionizes it, leaving a trail of plasma.

As the comet gets closer to the sun and is heated more and more, it will produce a complex and dynamic atmosphere of gas, dust, and plasma called a coma, full of outbursts and jets. “We’ll get a full history of the time evolution of [67 P’s] plasma environment. A comet’s plasma is completely different to that of a planet,” Carr says.

In November, potentially the most difficult part of the mission will take place when Rosetta dispatches a lander called Philae onto the comet’s surface. Once it touches down, the lander will fire a harpoon into the comet to hold it in position; the comet itself doesn’t provide enough gravity. The lander has the ability to extract a sample of material from several centimeters below the comet’s surface. This has huge interest for researchers because comets, which form and live most of their lives in the outer reaches of the solar system, are pristine relics of the stuff planets were originally made of. Getting hold of some of that material will help scientists understand the nature of the planets we see today.

It will also help answer questions about what influence comets might have had on the evolution of our own planet. Earth seems to have more water than models would suggest, and one theory is that early in its life, comets rained down on its surface, depositing water. Rosetta’s lander has the ability to measure the relative abundances of hydrogen and its isotope deuterium in the water on the comet. If that isotope ratio is similar to that in the water on Earth, it will support the idea that comets put it there. Some researchers also believe comets seeded Earth with organic compounds that later produced life. The lander will look for such complex organic molecules.

Carr says that many have been working on this project for decades. At the start, “it seemed so far off. I never thought this day would come.”